The present invention relates to a solid state image pick-up device in which either a shutter function or a contour enhancing function can be selected at will.
In a conventional method for improving the quality of a television image, there has been proposed a method for effecting the contour enhancement of the image. According to the known method, the contour enhancement in a horizontal direction in parallel with the scanning line can be achieved by a relatively simple circuit construction. However, in order to perform the contour enhancement in a vertical direction, it is necessary to use an expensive and large device such as a precise delay line and a mass-storage memory.
In the known solid state image pick-up device, in order to emphasize the contour of the image in the vertical direction, it is necessary to calculate the difference between image signals derived from adjacent scanning lines. In the known solid state image pick-up device, since it is impossible to simultaneously obtain adjacent plural scanning lines, use may be made of a delay line having a delay time corresponding to one horizontal scan period. Since the delay line requires a high precision, it is very expensive. Moreover, since, in the case of performing the interlace scanning operation, the signal obtained by the delaying operation for one horizontal scanning period is the image signal belonging to the same field, the signals thus obtained are not from adjacent scanning lines, but from every other line. Therefore, the thus obtained contour signals in the vertical direction include a lot of low frequency components and thus sufficient contour enhancement cannot be achieved.
In order to obtain the adjacent scanning line signals the case of effecting the interlace scanning operation, there is a method for storing the image signals of one field in a field memory. In this case, the contour signals can be obtained over a high frequency range. However, since the field memory requires an extremely large mass storage, the entire pick-up device is liable to be more expensive and larger.
The solid state image pick-up device of the kind mentioned above operates under the so-called XY address system and has an electronic shutter function. Such a solid state image pick-up device can be manufactured by utilizing the well-developed MOS-IC technique and has been generally applied to small size video cameras.
FIGS. 1A and 1B illustrate an example of a known solid state image pick-up device. FIG. 1A shows schematically the whole construction of the device and FIG. 1B is a cross section depicting the construction of a single picture element. The image pick-up device comprises a number of picture elements 1-11, 1-12 . . . ; 1-21, 1-22 . . . ; . . . arranged in a matrix form and formed in a p-type semiconductor substrate 1. Each of the picture elements comprises a MOS transistor having a source 2 and a drain 4 formed in a main surface of the substrate 1 and a gate 3 formed on an insulating layer applied on the main surface, and a n.sup.+ p photodiode 5 constituted by a junction between the p-type substrate 1 and n.sup.+ source 2. In the source 2 is stored a signal charge whose amount corresponds to the incident light amount. To the gates 3 of the MOS transistors are applied vertical scanning pulses from a vertical shift register 6, while to the drains 4 of MOS transistors are connected to horizontal selection switches 7-1, 7-2, . . . comprising MOS transistors and horizontal scanning pulses are applied to the switches from a horizontal shift register 8 so as to connect selectively the drains 4 of the MOS transistors to a video output line V.L. The horizontal scanning pulse has a frequency f.sub.H of 15.75 KHz and the vertical scanning pulse has a frequency fv of 30 Hz.
When the vertical scanning pulse is applied to the gate 3 of the MOS transistor constituting a picture element and, at the same time, the drain 4 of the related MOS transistor is selected by a horizontal selection switch 7 driven by the horizontal scanning pulse, the signal charge stored in the photodiode 5 is read-out onto the video output line V.L. At the same time, the photodiode 5 is reverse-biased or reset by a supply source V.sub.D, so that the charge storing operation is started again.
In the known solid state image pick-up device explained above, the reading out operation and the reset operation are effected simultaneously, the charge storing period, i.e. the exposure time corresponding to a shutter open time is fixed to 1/fv=33.3 m sec. and cannot be made shorter than 1/fv.
In general, in the case of picking-up a moving object, it is necessary to shorten the shutter open time in order to obtain a sharp image of the object. However, in the known pick-up device, since the shutter open time is fixed, when the moving object is picked-up, the image sharpness is decreased and the image quality is greatly deteriorated. In order to obviate such a drawback, it has been proposed to provide a high speed rotary shutter in a video camera or to ignite a stroboscopic lamp during a vertical blanking period. However, such a solution requires special device, so that the camera is liable to be complicated in construction, large in size, heavy in weight and expensive in cost, and further usage of such a camera is limited in various ways.
As explained above, in order to improve the quality of the image reproduced by the image signal derived from the solid state image pick-up device, it is preferable to effect the vertical enhancement and the electronic shutter speed control. However, in practice, it is very difficult to carry out both of the above measures simultaneously. The inventors have recognized the fact that the image quality can be improved considerably, even by effecting either of the two measures depending upon the object to be picked-up. For instance, if an object is moving at a high speed, it is preferable to shorten the shutter open time, but if an object that is standing still is picked-up, the image quality can be improved materially by effecting the vertical enhancement, while the shutter speed is fixed to one field period. Therefore, a solid state image pick-up device in which either the vertical enhancement or the shutter speed control can be selectively effected is required.